CAN (Controller Area Network) bus has been developed for a wide variety of distributed industrial systems with automotive applications being probably the most prominent field of application. CAN is a multi master architecture which means that each entity connected to the bus (also referred to as a node) is able to send and receive messages, but not at the same time. This is achieved by providing each message with an identifier which also defines the priority of the corresponding message. In order to avoid collisions of messages sent at the same time by different nodes, an arbitration phase precedes each phase during which “payload” data is sent. During the arbitration phase the node with a message of the highest priority is designated to transmit its message whereas all the other nodes enter a listening mode. One aspect limiting the data rate originates from the arbitration phase. During that phase the cycle length of the data rate has to be larger than the time it takes for the signal to travel back and forth from a respective node to a node lying farthest away within the network. In other words, the length of the lines within the network is one factor limiting the data rate.
Further factors limiting the data rate are signal integrity and capacitive loads which affect signal propagation during the arbitration phase, as well as the propagation delay of signals on the conductors. In order to circumvent those limiting factors, CAN FD (CAN with flexible data rate) has been conceived. CAN FD allows for sending data with a slower rate during the arbitration phase, after which the data rate may be increased.
However, the use of common CAN transceivers introduces the following problem. Depending on the network quality, ringing, i.e. an unwanted oscillation of a voltage or current, with typical ringing times on the order of many hundreds of nanoseconds occurs at every switching between a dominant bit and a recessive bit, wherein the dominant bit corresponds to a state of the differential lines carrying substantially different voltages and is interpreted as a logical 0 and the recessive bit corresponds to a state of the differential lines carrying substantially the same voltages and is interpreted as a logical 1. The reason for the ringing can be seen in that the CAN transceivers are basically binary switches, which are either switched on (during the transmission of a dominant bit) or switched off (during the transmission of a recessive bit). The ringing caused by the transition from a dominant state to a recessive state disturbs the actual signal on the bus and impedes a fast and accurate detection of the state of the bus (i.e. a determination whether a dominant bit or a recessive bit is present on the data bus) in a potential receiving node. This effect may also limit the maximally possible data rate. In order to allow a proper communication within the CAN network, those issues are taken into account when the maximal data rate using ordinary CAN transceivers is set. The maximal data rate is so to speak adapted to the integrity of the signals which in practice results in its appropriate limitation.